Anesthetic drugs which, when properly administered, induce loss of awareness, are often used for painful and serious medical procedures such as surgical operations. A general anesthetic, when properly administered, will cause a progressive depression of the central nervous system so that the patient loses consciousness. A local anesthetic, however, only affects sensation at the region to which it is applied.
Generally, the patient, prior to a surgical operation, is anesthetized by a specialized medical practitioner (“anesthesiologist”) who may be a Board Certified physician, or a specially trained nurse anesthetist. One or more volatile inhalational liquids or gases may be administered (e.g., nitrous oxide, methoxy flurane, sevoflurane, isoflurane, desflurane, ethylene, cyclopropane, ether chloroform, halothane, etc.). Certain desirable anesthetic gases such as Ciboflorane® (Abbott Lab) may sometimes not be used because of their cost. Alternatively, nonvolatile drugs may be administered by injection or intravenous infusion (e.g., flumazenil, thiopentone, Retamine, remifentanyl, midazolam, pentothal, propofol, evipal procaine and etomidate® (Abbott)). The objectives of general anesthesia administered prior to a surgical operation, may include:    a) blocking the patient's movements and relaxing the patient's muscles to prevent involuntary reflex muscle movements which may interfere with the operation;    b) preventing the patient from being aware (i.e., loss of consciousness, or sedation) during the operation;    c) preventing the patient feeling pain (i.e., loss of sensation, or analgesia) during the operation; and    d) preventing the patient from remembering intra-operative events or discussions (i.e., amnesia).Furthermore, the anesthesia should not lower blood pressure to a dangerous extent (e.g., below 50 mm Hg for mean arterial pressure (MAP)).
These objectives of general anesthesia may often be attained by separate administration of hypnotic or sedative, analgesic and amnesic agents, in accordance with the clinical judgment of the managing anesthesiologist evaluating the apparent state of the patient and a variety of vital signs.
In order to monitor the “anesthetic depth” or “plane of anesthesia” of the patient, a skilled anesthesiologist looks at the vital signals of the patient (e.g., breathing, blood pressure, etc.) to determine if more, or less, anesthetic is required. Often he/she looks into the patient's eyes to determine the extent of the dilation of the pupils as an indication of the level (or depth) of the effect of the anesthesia. Complete reliance on the availability, skill and attention of the anesthesiologist presents problems in some situations. In addition, respiration may be artificially controlled (e.g., by a respirator) and/or medications may block or alter useful autonomic signs. In the absence of graded neurological reflexes, the depth of suppression of brain activity related to awareness often may not be accurately gauged. The mute, paralyzed patient cannot report the experience of pain. Furthermore, pain cannot be reliably inferred from vital signs since they may be blocked by the presence of medications. In some operations (e.g., heart surgery), the head is covered so that the patient's eyes cannot be viewed and pupillary dilation is not apparent. No reliable estimate may then be made of the possibility that the patient may be aware of environmental events, experience pain and/or be able to store and retrieve memories about unpleasant experiences. Furthermore, during prolonged operations (e.g., 10 to 15 hours or more), the attention of the anesthesia nurse or anesthesiologist may not be constant.
Also, at times, an anesthesiologist may not be available (e.g., in emergency or battlefield situations). Similarly, in isolated geographic locations, it may be impractical to move a patient requiring an operation to a hospital where an anesthesiologist would be available. However, a physician or surgeon may be able to perform a required operation if there were some way to effectively and safely anesthetize the patient.
U.S. Pat. No. 2,690,178 to Bickford purports to describe an automatic system for applying anesthetic to a patient while monitoring the patient's brain waves to monitor the effects of the anesthetic. Bickford used an integrated potential output of the cortex to judge the efficacy of the anesthetic. (See also, U.S. Pat. Nos. 4,280,494 and 4,533,346 to Cosgrove et al. entitled “System for Automatic Feedback-Controlled Administration of Drugs”). The EEG measure used is an “EEG power response” (i.e., a total power output of the brain). However, the use of the single measure of integrated cortex output as described in the Bickford and Cosgrove patents may not provide a reliable control signal for applying a general anesthetic. Different anesthetics have different impacts on power output and several may actually cause an increase in a power detected by a cortical EEG. Furthermore, in some instances, the nature of the power detected changes depending upon electrode position. In addition, not only do different anesthetics have different effects upon the EEG, but those effects may vary from patient to patient as a consequence of different pre-operative medications and/or different biochemical sensitivities.
The following patents which describe methods and apparatus for monitoring and/or controlling the provision of anesthetic to patients are hereby expressly incorporated by reference: U.S. Pat. No. 6,315,736 to Tsutsumi et al.; U.S. Pat. No. 6,317,627 to Ennen et al; U.S. Pat. No. 6,016,444 to E. R. John; U.S. Pat. No. 5,699,808 to E. R. John; U.S. Pat. No. 5,775,330 to Kangas et al.; U.S. Pat. No. 4,557,270 to E. R. John; U.S. Pat. No. 5,010,891 to Chamoun; and U.S. Pat. No. 4,869,264 to Silberstein.